1. Field of the Invention
The present invention relates to coating method and apparatus, in particular to coating method and apparatus for use in manufacturing photosensitive materials and magnetic recording media.
2. Description of the Related Art
Photosensitive materials and magnetic recording media are manufactured through a coating step of forming a coating film by applying a prescribed coating liquid, such as magnetic liquid, to a continuously running sheet substrate strip (hereinafter referred to as “web”). In recent years, magnetic recording media such as magnetic recording tape particularly for broadcasting or computer use have been rapidly improved in capacity and recording density, and there has been needed a coating technology capable of providing a magnetic layer which is extremely thin and of which surface is smooth.
Coating apparatus for applying a coating liquid to the surface of a web include, for example, apparatus of types such as roll coater, gravure coating, roll coating plus doctor roll, extrusion coating, and slide coating; in recent years, apparatus of extrusion type have been commonly used.
One method which employs an extrusion type of apparatus and in which the tip of a coating head is pushed against a web, as described in Japanese Patent Application Publication No. 58-109162, has been commonly used in the field of manufacturing magnet recording media, since a thin and uniform coating layer can be obtained just by increasing the fluid pressure at the tip of the coating head utilizing the web tension, thereby eliminating the air accompanying the web. However, even in the method of applying a single coat of coating liquid using this type of coating head, the reduction in coating thickness is limited.
As a further method for forming a much thinner coating layer, there is a method in which a very thin coating layer can be formed by applying a coating liquid to a web to excess using a web-pushing type of extrusion coater and then scraping-off the excess coating liquid with a blade disposed on the downstream side of the extrusion coater, as described in Japanese Patent Application Publication No. 7-287843. In this case, the device which applies a coating liquid to a web to excess is not necessarily limited to the web-pushing type of extrusion coater, but coaters such as roller coater type coater, gravure coater and extrusion coater equipped with a back-up roll can be used.
In the method using the conventional scraping-off type of extrusion coater shown in FIG. 8, however, since the coating liquid recovered by scraping-off, with a blade 6, the excess amount of coating liquid having been applied to a web 5 at a coating head 3 has been once exposed to the atmosphere, the solvent contained in the coating liquid has been volatilized, thereby changes are caused in the fluid physical properties such as viscosity and solid content concentration. Thus, the method has a disadvantage in that, since the use of the coating liquid, as it is, of which viscosity has been changed adversely affects the amount of the coating liquid to be scraped-off or the final thickness of the coating film after being scraped-off with the blade 6, the recovered coating liquid must undergo treatment for adjusting its physical properties such as viscosity and solid content concentration in a coating liquid adjusting tank 8 before reused. In addition, since the recovering line for recovering the coating liquid is opened to the atmosphere, dust from the outside of the recovering line is likely to be included in the recovered coating liquid. Thus, the method also has a disadvantage in that dust cannot be fully removed even if the recovered coating liquid is filtered through a filter 9 when pumping the recovered coating liquid from a supply tank 1 to the coating head 3 with a pump 2.